Pools

multiple targets for a single rdr/nat rule or route-to option
NATing to multiple public addresses or netblocks:

nat on kue0 from 10.0.0.0/8 to any -> \ 
  { 1.2.3.4, 2.3.4.5, 3.4.0.0/16 } round-robin

load-balancing through several interfaces:

pass out on kue route-to { kue0, kue1, kue2 } round-robin \ 
  keep state

various allocation schemes:
round-robin loops through the targets sequentially
source-hash hashes the source address so the same source address is always mapped to the same target
bitmask replaces the network part of the source address, e.g. a.b.4.5 is mapped to x.y.4.5
random selects a random target
stateful filtering ensures the same mapping is applied to all packets of a connection